The inception of the first polymer-based banknotes in Australia in 1988 (see FIG. 1A) has provided a new perspective to the global economic security. The new polymer banknote material, named GUARDIAN, was developed jointly by the Reserve Bank of Australia, Commonwealth Scientific and Industrial Research Organization (CSIRO), and the University of Melbourne. Besides GUARDIAN as the currently most used material, a few other types of polymer banknotes have been developed, namely TYVEK by DuPont and American Bank Note Company in 1980s and DURANOTES by Canada's AGRA and the US Mobil Chemical.
At present, there are at least 30 different denominations totaling >3 billions polymer-based banknotes that are currently circulating in ˜30 countries, which accounts for ˜15% of the total independent countries in the world. Additional countries, including European Union, Canada, Switzerland, and India, have plans to issue polymer banknotes starting from 2011 and beyond.
Counterfeiting is one of the top issues of banknotes. Taking United States as an example, its currency is known to be vulnerable to counterfeiting because its design was created in 1920s and it is an international currency valued all over the world. With rapid improvement in printing technology, such as high resolution scanners and printers, there has been a growing number of counterfeiting events. In 1995 only, over 360 million counterfeit notes were confiscated, and it is estimated that more than billion dollars were undetected. This counterfeited money is used for arm purchase, drug trade and terrorist activity, which is a serious threat for homeland security as well as international welfare.
Polymer bank notes have provided a strong barrier to counterfeiters in many countries. For example, since the 20-Mexican peso polymer banknotes introduction in 2002, Mexico has seen a dramatic decrease of the number of detected counterfeit 20-peso banknotes by 96% in the following years. However, the cost of polymeric banknote manufacturing at present is still higher than that of paper-based banknotes. Effective yet inexpensive counterfeiting techniques with polymer materials are still lacking.
Secure, durable, and printable are the three technical criteria for Central Banks to purchase banknote materials. Security as the universal philosophy requires the features to be difficult to be counterfeited, yet they have to be simple enough for the general public to recognize. Hence, the transformation proceeding of paper-based banknotes to polymer-based banknotes highly rely on the degree of difficulty for counterfeiting, robustness for long-term use and printing cost as some of the key determinative aspects. In the case of U.S., ˜1700 billions of US dollars are circulating in the world, according to the Federal Reserve by October 2010. The cost of printing a paper-based banknote is ˜$0.04, while the circulation time of current paper/cotton-based banknotes is only ˜18 months on average, in comparison to the 4-year life time of polymeric banknotes. The estimated potential impact of transforming all current paper-based U.S. banknotes into polymer-based ones is of 0.68 billion to 68 billion. As a result, there is a great demand in developing advanced anti-counterfeiting features specifically for polymer-based banknotes.
Owing to the unique material properties and manufacturing techniques of the polymer thin film, new security features were introduced into the polymer-based banknotes that were not available to the paper-based ones. For instance, GUARDIAN banknote employs optical transparent windows with embossed microscale features which are incorporated within the polymer substrate, as shown in FIG. 1B. Two-dimensional (2D) optical devices, in the form of diffraction gratings (G-SWITCH, 102), iridescent bands (IRISWITCH, 103), and shadow images (SHAD H2O SWITCH, 104), can also be introduced into the optically transparent window (WINTHRU, 101) through embossing techniques. While the introduction of these new anti-counterfeiting features into polymer-based banknotes has greatly increased the currency security, the full potential of anti-counterfeiting methods suitable for polymer substrate has not yet been realized.